In mass spectrometry, high voltage RF power supplies are widely used for supplying potentials to different ion optical devices, such as mass filters, collision cells, transfer multipoles, etc. Typically, such RF power supplies provide two complementary phases of RF voltage with amplitudes in the range of 100V peak-to-peak to 1 kV peak-to-peak, at frequencies between 0.3 and 3 MHz measured on one phase relative to ground.
From a practical perspective, such RF power supplies are often built on a resonant tank principle. The output inductance of an RF transformer in the RF power supply and the self-capacitance of the ion optical device (as defined at the input) present a resonant tank. Usually, the RF power supply has only one RF transformer with a quality factor (Q) between 100 and 200, a transformation ratio (n) between 30 and 50 and a supply voltage of 24 VDC or 48 VDC. This structure is advantageously simple and keeps the power consumption of the RF stage in the power supply low. The resonant frequency of the tank circuit is described by the well-known formula,
      f    =          1              2        ⁢        π        ⁢                  LC                      ,
where L is the inductance of the secondary (output) winding of the RF transformer and C is the sum of self-capacitances of the ion optical device and the secondary winding.
Existing RF transformers are mostly built as an air-core coil that allows, by use of an appropriate material for a coil former, to keep the tank resonant frequency stable in view of normal temperature variation. It is relatively unusual for the RF transformer, supplying an ion optical device, to be wound on a ferrite or metal powder core. Whilst such cores provide some advantages, such as compactness and low production costs, there may be significant power losses in such cores and they may cause a relatively high temperature dependency of the resonant frequency.
The RF power supply does not typically provide an output with fixed parameters. The detection mass range in modern mass spectrometers used in life sciences is wide and may vary between 50 Da to 50 kDa or even greater. This range depends upon the most limiting ion optical device within the mass spectrometer. The ratio between the highest to lowest mass measurable in one analysis cycle typically does not exceed 20. As a result, the whole mass range to be analysed is normally divided into a plurality of sub-mass ranges. This is achieved by changing the RF and DC voltages supplied to the ion optical devices for each sub-mass range measurement. Sometimes, it is more effective to change the frequency of the RF voltage simultaneously.
In theory, it is possible to connect additional frequency-setting capacitive or inductive reactances in parallel with the secondary winding of the air-cored RF transformer. An example of such an embodiment is shown in FIG. 1a. This comprises an RF generator 10 which provides an input to a transformer 20. The transformer comprises a primary side with a primary winding 21 and a secondary side with a secondary winding 22. In parallel with the secondary winding 22, there is an inductor 30 of inductance Lext, which is controlled by a first switch 35. Also in parallel with the secondary winding 22 is a capacitor 40 of capacitance Cext, which is controlled by a second switch 45. A capacitance 50 represents the self-capacitance of the ion optical device to which the transformer 20 provides its output. Transformer 20 is air-cored in this case.
Referring next to FIG. 1b, there is shown an alternative theoretical embodiment of an RF power supply according to a conventional design. Like the embodiment shown in FIG. 1a, this comprises an RF generator 10. Where the same components are identified, identical reference numerals have been used. A magnetic-core based transformer 120 comprises a primary side with a primary winding 121 and a secondary side with a secondary winding 122. The primary winding 121 and secondary winding 122 are inductively coupled via a magnetic core 123. As with FIG. 1a, there is provided an inductor 30 controlled by a first switch 35 and a capacitor 40 controlled by a second switch 45, in parallel with the secondary winding 122. These are provided in parallel with the output to the ion optical device, represented by capacitor 50. In addition, there is provided a second inductor 130 (of inductance Lext′) controlled by a third switch 135 and a second capacitor 140 (of capacitance Cext′) controlled by a fourth switch 145, which are provided in parallel with the primary winding 121 of the transformer 120.
These apparently straightforward methods for changing the resonant frequency of the tank circuit at the output of the RF transformer have problems when implemented. For the embodiment of FIG. 1a using an air-core transformer 20, there are two main technical problems. The first switch 35 and second switch 45 are required to cope with high voltages and currents, but without adding significant intrinsic capacitance to the resonant tank. For electro-mechanical switches, the cost, reliability and size needed to match these requirements is not easy. In contrast, semiconductor switches have a large output capacitance, which may exceed the capacitance present without the additional components. Moreover, in order to avoid high power losses in the second inductor 30, this inductor is desirably implemented with an air core. This may mean that the inductor 30 is as big as the RF transformer itself.
For the magnetic-core based transformer embodiment shown in FIG. 1b, the commutation of reactances on the secondary side of the transformer has similar problems as described above with reference to the embodiment of FIG. 1a. On top of these difficulties, the commutation of reactances on the primary side of the transformer causes further problems. The second inductor 130 and second capacitor 140 desirably have very low intrinsic resistance, preferably 20 mΩ or less, in order to keep the quality factor of the resonant tank sufficiently high.
The output reactances are reflected to the primary side, with a proportionality factor of n2,
            n      2        =                  (                              N            p                                N            s                          )            2        ,
where Np and Ns are the numbers of turns in respect of the primary winding 121 and secondary winding 122 respectively. Inductances are reflected and become lower by a factor of n2 and capacitances are reflected and become higher by the same factor. Thus, the second capacitor 140 desirably has a low loss at a relatively high capacitance value (up to hundreds of nF) and very low Equivalent Series Resistance (ESR). The inductance of the second inductor 130 also desirably is kept low (sometimes less than 100 nH) and might become lower than the leakage inductance of the RF transformer itself. In addition, the output current of the RF transformer is reflected to the primary side multiplied by a factor of n. This may reach a level of tens of Amperes.
In view of this, the addition of reactances on the primary side of the RF transformer adds at least two further technical difficulties. It is difficult to find high-current inductors or capacitors with high RF quality factors to meet the requirements described above. Moreover, it is difficult to build a magnetic-core based RF transformer with very low leakage inductance. On this basis, there are significant practical challenges to changing the resonant frequency of a high voltage resonant tank using such an RF transformer by simply connecting reactances in parallel. Commercial approaches must set a compromise between minimising power losses and minimising costs. Designing RF transformers for such power supplies to meet both these requirements remains a significant challenge.